MIDDLETON, Mass.—An internal investigation has determined that an explosion at a Middleton chemical plant that sent four workers to the hospital was caused when two valves were left open, allowing a building to fill with flammable vapors that were ignited by an internal ignition source.
The information on the investigation released by Bostik Inc. on Tuesday did not say why the valves were left open or what specifically touched off the blast in March.
A company spokeswoman tells The Salem News there were no open flames in the room and the vapors could have been ignited by static electricity or a machine.
The explosion happened during the manufacture of liquid adhesives.
All four workers have since returned to work.
State Fire Marshal Stephen Coan says his agency's investigation is nearly complete.